Vehicles such as automobiles are equipped with air-conditioning systems for maintaining a good temperature environment in a passenger compartment, regardless of the outside air temperature and the like. The vehicle air-conditioning system normally includes several outlets in a panel in front of a driver's seat and a front passenger seat. The vehicle air-conditioning system adjusts a discharge temperature to a target discharge temperature as appropriate, so as to maintain the temperature in the passenger compartment at an optimum temperature.
The adjustment of the discharge temperature is performed by adjusting the opening of an openable-closable air mixing door disposed between an evaporator and a heater core within an air-conditioning duct, and adjusting the mixing ratio between hot air (heated air) and cool air (unheated air). The air mixing door provided upstream of the heater core has the function of dividing air passing through the air-conditioning duct into heated air and unheated air.
As a means for precisely adjusting a discharge temperature to a target discharge temperature at outlets of a vehicle air-conditioning system, a vehicle air-conditioning control device disclosed in JP-A-56-82626 is known, for example. This vehicle air-conditioning control device includes a temperature sensor provided at an outlet, and compares an actual discharge temperature detected by the temperature sensor with a target discharge temperature set by a control system, and adjusts the opening of an air mixing door so that the actual discharge temperature comes close to the target discharge temperature.
When the actual discharge temperature is feedback controlled to the target discharge temperature by adjusting the opening of the air mixing door in the vehicle air-conditioning control device, as shown in the above 56-82626 publication, the following problems will arise:
The temperature of a heater core on the high-temperature side depends on the water temperature in an engine, and is greatly different from the temperature of an evaporator on the low-temperature side. That is, there is a great temperature difference between cool air supplied from the evaporator and hot air passing through the heater core under the dividing operation of the air mixing door. When the opening of the air mixing door is changed based on the feedback control to divide air passing from the evaporator to the heater core into heated air (hot air) and unheated air (cool air), the discharge temperature is greatly changed relative to the change in the opening of the air mixing door. That is, since the amount of air heated by the downstream heater core and the amount of air as cool air are determined solely by the air mixing door, a great temperature change occurs in an adjustment to a target discharge temperature, resulting in rough adjustment. In other words, the feedback control in the related art chiefly divides air into heated air and unheated air solely by the air mixing door, and thus cannot perform fine adjustment of temperature, leaving a problem that precise control of discharge temperature cannot be performed.
In this connection, the inventor has previously presented in Japanese Patent Application No. 2003-315771 (dated Sep. 8, 2003) corresponding to U.S. application Ser. No. 10/937,582, filed Sep. 8, 2004, now U.S. Pat. No. 6,945,323, a vehicle air-conditioning system which solves the above problem. This vehicle air-conditioning system is provided with a cool air adjusting door in addition to an air mixing door to increase precision in discharge temperature control. In other words, in addition to a cool air supply passage formed depending on the opening of the air mixing door, another passage through which cool air can also be supplied is provided so as to increase the amount of cool air. With this, when cool air is mixed with high-temperature hot air which is greatly different in temperature from the cool air, an appropriate amount of cool air can be supplied to increase precision in discharge temperature control.
However, the vehicle air-conditioning system in 2003-315771 leaves room for improvement as will be described below. With the structure of this vehicle air-conditioning system, when the opening of the air mixing door is small and close to a fully-closed state, there is little airflow passing through a heater core, and thus the discharge temperature is near the temperature of air passing through the cool air adjusting door. Therefore the adjustment of the cool air adjusting door only provides a small change in discharge temperature. This indicates a stable discharge temperature. On the other hand, as is typical in feedback control, the discharge temperature is compared with a target discharge temperature, and the position of the cool air adjusting door is adjusted so that the temperature difference falls within a predetermined range. Therefore, the cool air adjusting door can take any position in the range from a fully-closed position to a fully-open position. As a result, when the cool air adjusting door is opened, a first bypass passage formed by the air mixing door in a fully-closed state and a second bypass passage formed by the cool air mixing door in an open state are both open, resulting in a large amount of air supplied from an outlet. When the cool air adjusting door is closed, the second bypass passage is narrowed, resulting in a small amount of airflow supplied from the outlet.
With the above vehicle air-conditioning system, when the opening of the air mixing door is small, the amount of air discharged from the outlet is not steady, as described above. It is thus required for the vehicle air-conditioning system in the related art configured to control the opening of the air mixing door to bring a discharge temperature close to a target discharge temperature, to remedy rough temperature control due to a large temperature difference between hot air and cool air.
For these reasons, there is a desire for an improved air-conditioning system which can reduce a temperature change in adjustment for good precision, and stabilize the amount of air discharged from outlet even when the opening of an air mixing door is small.